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CHARGE TRANSPORT MECHANISMSIN ORGANIC MATERIALS
Michael R. WasielewskiDepartment of Chemistry and Institute for Nanotechnology
Northwestern University, Evanston, IL 60208-3113
+ -+ -
hν e-
NN
O
O
O
O
O
O
N
N N
NZn
hν e-hν e-
NN
O
O
O
O
O
O
N
N N
NZn
+ -
Energy Transfer, 21 ps
Electron Transfer, 7 ps
Energy Transfer, 21 ps
Electron Transfer, 7 ps
SUPEREXCHANGE: COHERENT ET
D B1 B2 B3 A
kET = k0 e-βr
HOPPING: INCOHERENT ET
D B1 B2 B3 A
kET = k0 (1/r)
Molecular Wire Behavior inp-Phenylenevinylene Oligomers
Charge Separation
WIRE N
O
O
N
O
O
C8H17
Donor Acceptor
RO
OR
RO
OR
RO
OR
R = 2-ethylhexylWire = 1.
2.
3.
4.
5.
Charge Recombination
Davis, W. B., W. A. Svec, Ratner, M.A., Wasielewski, M.R., Nature, 1998, 396, 60-63.
Rylenes Absorbing the Entire Solar Spectrum• Chemically robust, easily oriented due to their rectangular shape• Excellent chromophores as well as electron donors and acceptors• Strong tendency to π stack in a variety of solvents and in the solid
R = 3,5-di-t-butylphenoxy
N
O
O
R
R
N(CH2)nNN
O
O
O
O
R
R
PDI 5PMI (n=0), 6PMI (n=1)
5PDI (n=0), 6PDI (n=1)
N
O
OR
R
PMI
NN
O
O
O
O
N
N
(CH2)n
(CH2)n
R
R
R
R
NN
O
O
O
O
QDI
Acceptor-Bridge-Donor Series
N N
RO
ORO
O
O
O
N N
RO
ORO
O
O
O
N N
RO
ORO
O
O
O
N N
RO
ORO
O
O
O
N N
RO
ORO
O
O
O
N S
N S
N S
N S
N S
Michael Ahrens
p-Phenylene Oligomer Properties
Eox. (vs. SCE) UV-vis250
350
2.4 V
1.85 V
1.60 V
1.47 V
~1.4 V
1.34 V
Meerholz, K., Heinze, J., Electrochimica Acta, 1996, 41(11/12), 1839-1854.
UV-Vis Spectra andSpectroelectrochemistry
300 400 500 6000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Abs
orba
nce
(nor
mal
ized
)
Wavelength (nm)
1 2 3 4 5
400 500 600 700 800
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Wavelength (nm)
∆A
NN
O
O
O
O O
O
C8H17C8H17
potential = 0 eVpotential = -0.6 eV
E0X= 0.83 eV vs. SCE
PDI Properties:RobustEred = -0.53 V Eox. = 1.7 VEs = 2.25 eVFluorescence Quantum yield ~ 1Intense Radical Anion Absorption
Daub, J., et. al., J. Phys. Chem. A, 2001, 105, 5655-5665.
500 600 700 800-0.10
-0.08
-0.06
-0.04
-0.02
0.00
0.02
0.04
0.06
0.08
∆ A
Wavelength, nm
3 ps 400 ps
Stimulated emissionfrom 1*PDI
Transient Spectroscopy of PTZ+-Ph2-PDI-
Observation of PDI radical anionat 720 nm
Stimulated emission Decay from 1*PDI at 620 nm
NN
OR
RO
O
O O
O
C8H17NS
0 100 200 300 400 500 600 700 8000.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
∆ A
Time, ps
τ = 202 ps
Louise Sinks
Distance Dependence of Charge Separation forPTZ-(Ph1-5)-PDI
β = 0.46 Å-1
12 14 16 18 20 22 24 26 28 30 32107
108
109
1010
1011
n=4
n=5
n=3
n=2
n=1k C
S (
s-1)
rDA (Å)
Energy Levels Relevant to Charge Separation
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
n = 5n = 4n = 3
n = 2
PTZ-Phn-3*PDI
PTZ-Phn+.-PDI-.
PTZ+.-Phn-PDI-.
PTZ-Phn-PDI
PTZ-Phn-1*PDI
Ene
rgy
(eV
)n = 1
n = 5n = 4n = 3n = 2n = 1
PDI Radical Anion Decay and 3*PDI Formation
NN
OR
RO
O
O O
O
C8H17NS
50 100 150 200 250 300
0.0
0.2
0.4
0.6
0.8
1.0
∆A (N
orm
aliz
ed)
Time (ns)
723 nmτ = 21 ns
50 100 150 200 250 300
-0.015
-0.010
-0.005
0.000
0.005
0.010
∆A
Time (ns)
450 nmτ = 20 ns
Singlet and Triplet Radical Ion Pair States
B(2J)
(S)
(T+1)
(T0)
Splitting of Triplet Levelswith Applied B-Field
(T-1)
2J
Ener
gy (a
rbitr
ary
units
)
Magnetic Field Strength
∑−∆
ΨΨ=∆=
nnRP
nelRPST E
VEJ
2
2
VRPS-S0
VRPT-T1
hfc
3(D-A)
VRPS-S1
3(D+-A-)1(D+-A-)
D-A
1*D-A
Ene
rgy
(arb
itrar
y un
its)
[ ] ( )∑∞
= ⎥⎥⎦
⎤
⎢⎢⎣
⎡ ++∆−−=
0
22
4exp
!exp
412
n B
TCR
n
BDAET Tk
nGnSS
TkVk
λωλ
πλπ ηη
Spin-Spin Interactions in Wire-Like Systems
0 50 100 150 2000.7
0.8
0.9
1.0
1.1
1.2
1.3
Rel
ativ
e Tr
iple
t Yie
ld (T
/T0)
Magnetic Field (mT)0 2 4 6 8 10 12
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
Rel
ativ
e R
P Y
ield
(RP
/RP 0)
Magnetic Field (mT)
PTZ+. Ph3 PDI-. PTZ+. Ph4 PDI-.
2J = 31 mT 2J = 6.2 mT
Emily Weiss
Distance Dependencies in Wire-Like Phn Systems
Spin-Spin Exchange Interactionin PTZ+.-(Ph)n-PDI-.
Charge Recombination Rate toTriplet PDI
β = 0.67 Å-1
16 18 20 22 24 26 28 30 321
10
100
1000
n= 5
n= 4
n= 3
2J (
mT
)
rDA (Å)
n= 2
12 14 16 18 20 22 24 26 28 30 32106
107
108
109
n= 5n= 4
n= 3
n= 2
k CR (s
-1)
rDA (Å)
n= 1
Energy Levels Relevant to Charge Recombination
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
n = 5n = 4n = 3
n = 2
PTZ-Phn-3*PDI
PTZ-Phn+.-PDI-.
PTZ+.-Phn-PDI-.
PTZ-Phn-PDI
PTZ-Phn-1*PDI
Ene
rgy
(eV
)n = 1
n = 5n = 4n = 3n = 2n = 1
PTZ – (FL)n – PDI
NN
O
O
O
O
O
ONS
n
C6H13C6H13
Eox (FL)n n = 1-4: ~1.3 V vs. SCE (in DCM)
Eox (PTZ) = 0.8 V vs. SCEEox (PDI) = -0.5 V vs. SCE
300 400 500 6000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Abs
orba
nce
(nor
mal
ized
)
Wavelength (nm)
n = 1 n = 2 n = 3 n = 4
PTZ
FL1
FL2
FL3
FL4 PDI
Randall Goldsmith
Charge Recombination through Oligofluorenes
100 200 300 400 500
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
∆ (N
orm
aliz
ed)
nanoseconds250 500 750 1000 1250 1500
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
∆A
nanoseconds100 200 300 400 500
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
∆A
nanoseconds100
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
200 300 400 500
nanoseconds
∆A
n = 1 n = 2 n = 3
τCR = 32 ns τCR = 266 ns τCR = 51 ns τCR
Kinetic Traces monitoring the formation of 3*PDI at 455 nm
n = 4
= 22 ns
-200 0 200 400 600 800 1000 1200 1400 1600
0.6
0.8
1.0
1.2
1.4
1.6
Rel
ativ
e Tr
iple
t Yie
ld (T
/T0)
Magnetic Field (mT)0 100 200 300 400 500
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
Rel
ativ
e Tr
iple
t Yie
ld (T
/T0)
Magnetic Field (mT)0 20 40 60 80
0.80
0.85
0.90
0.95
1.00
1.05
1.10
Rel
ativ
e Tr
iple
t Yie
ld (T
/T0)
Magnetic Field (mT)
n = 2 n = 4n = 3
2J = 240 2J = 40 2J = 6 mT
Magnitude of the Spin-Spin Exchange Interaction in PTZ+.-(FL)n-PDI-.
Distance Dependencies in Wire-Like FLn Systems
Spin-Spin Exchange Interactionin PTZ+.-(FL)n-PDI-.
Charge Recombination Rate toTriplet PDI
20 22 24 26 28 30 32 34 36 38 401
10
100
1000
n = 4
n = 3
2J (
mT)
rDA (Å)
n = 2
10 15 20 25 30 35 40106
107
108
n = 4
n = 3
n = 2k C
R (s
-1)
rDA (Å)
n = 1β = 0.23 Å-1
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
n = 1-4
PTZ-FLn-3*PDI
PTZ-FLn+.-PDI-. PTZ+.-FLn-PDI-.
PTZ-FLn-PDI
PTZ-FLn-1*PDI
Ene
rgy
(eV
)
n = 5n = 4n = 3n = 2n = 1
Energy Levels Relevant to Charge Recombination
Charge Transport in Self-AssembledArtificial Photosynthetic Systems
hν e-
NN
O
O
O
O
O
O
N
N N
NZn
hν e-hν e-
NN
O
O
O
O
O
O
N
N N
NZn
ZnTTPEox= 0.82 eVEs = 2.06 eVΦf = 0.035
PDIEred= -0.53 VEs = 2.25 eVΦf = 1
T. van der Boom et al., J. Am. Chem. Soc.124, 9582-9590 (2002).
• The first two equations predict that• B1/2 = 1.1 mT.• The observed B1/2 = 68 mT is 60x larger.• This could be due to:
1) uncertainty broadening2) a distribution of 2J values
2/12 )1( ⎥
⎦
⎤⎢⎣
⎡+= ∑
kkkiki IIaB
21
22
21
2/1)(2
BBBBB
++
=
Magnetic Field Effect on Charge Recombination in(ZnTTP-PDI4)n Nanoparticles in Solution
B(2J)
(S)
(T+1)
(T0)
Zeeman Splitting of theTriplet Energy Levels
(T-1)
2J
Ene
rgy
Magnetic Field 0.0 0.1 0.2 0.3 0.4 0.50.92
0.94
0.96
0.98
1.00
1.02
Rel
ativ
e Tr
iple
t Yie
ld
Magnetic Field (T)
17 mT
68 mT
η≅⋅τ2/1B
Radical Ion Pair Distances within (ZnTPP-PDI4)n
1.48 nm
1.60 nm
1.78 nm
2.01 nm
2.27 nm
2.55 nm
2.84 nm
3.15 nm
3.46 nm
1.44 nm
3.78 nm
ZnTPP PDI
The fact that 2J = 17 mTindicates that the averageradical ion pair distance is21 Å. This corresponds toa PDI molecule 5 layersremoved from thelayer in which the initialradical ion pair wasgenerated.
hν e-
NN
O
O
O
O
O
O
N
N N
NZn
hν e-hν e-
NN
O
O
O
O
O
O
N
N N
NZn
The observed B1/2 = 68 mTImplies that khop = 4 x 1010 s-1
Symmetry-Breaking in the Excited State Leads to Quantitative Charge Separation in Dimers of 5PDI, a Green Chlorophyll a Mimic
• Strong absorber of 600-800 nm light.• EOX = 0.68 V and ERED = -0.76 V vs. SCE• The π-stacked cofacial chromophores
undergo symmetry breaking in theexcited state leading to quantitativecharge separation.
N N
N
N
O
O
O
O
5PDI
O O
N
O O
O
N
N
N
O
N
O O
O
N
N
N
300 400 500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
Abs
orba
nce
(AU
)
W avelength (nm )
cof-5PDI2 (to luene) 5PDI (to luene)
cof-(5PDI)2
J. M. Giaimo et al., J. Am. Chem. Soc. 124, 8530-8531 (2002).
cof-(5PDI)2
O
O N
O
O
O
N
N
N
O N
O
O
O
N
N
N
hν
τCS <150 fs τCR =880 ps
500 550 600 650 700 750 800-0.03
-0.02
-0.01
0.00
0.01
∆A
Wavelength (nm)
500 600 700 800
5PDI
cof-(5PDI)2
Combining Light-harvesting and Charge Separation in a Self-assembledArtificial Photosynthetic system Based on Perylenediimide Chromophores
NN
N
N
O
O O
O
N
N
O
O
O
O
O
O
N
N
O
O
O
O
O
ON
N
O
O
O
O
O
O
N
N
O
O
O
O
O
O
5PDI
PDI
PDI
400 500 600 700 8000.0
0.5
1.0
1.5
Abs
orba
nce
Wavelength (nm)
Chloroform Toluene
B. Rybtchinski et al., J. Am. Chem. Soc. 126, 12268-12269 (2004).
Small-Angle X-ray Scattering Structural Studies of 2 x 10-4M 5PDI-PDI4 in Toluene Solution
Guinier Plot PDF PlotScattering Intensity
0.1
100
Inte
nsity
q, Å-1
0.002 0.004 0.006 0.008
100
150
200
Rg = 18.1± 0.03
Inte
nsity
q20 20 40 60
0.0
0.4
0.8
P(r)
r, Å
Compound 1 Calculated dimer
Small-Angle X-ray Scattering Structural Studies of 2 x 10-4M 5PDI-PDI4 in Toluene Solution
Simulated AnnealingReconstruction of the Aggregate Shape
500 600 700 800
-0.06
-0.04
-0.02
0.00
0.02
0.04
0.06
∆A
Wavelength (nm)
0.6 ps 2.1 ps 9.1 ps 29 ps 306 ps 506 ps
A) 680 nm excitation
500 600 700 800-0.06
-0.04
-0.02
0.00
0.02
∆A
Wavelength (nm)
0.6 ps 0.9 ps 4.0 ps 20 ps 157 ps
B) 550 nm excitation
Transient Absorption Spectra of (5PDI-PDI4)2 in Toluene following Laser Excitation at 680 nm and at 550 nm
Energy Transfer, 21 ps
Electron Transfer, 7 ps
Energy Transfer, 21 ps
Electron Transfer, 7 ps
τCS = 7 psτCR = 420 ps
Summary:
•Wire-like behavior in electron donor-bridge-acceptor systems requires near-resonant energy level matching between a weakly-coupled electron donor and bridge.
•Using direct measurements of magnetic exchange interactions as a function of distance, the superexchange and hopping contributions to the overall electron transfer rate can be dissected.
•Photoexcitation of self-assembled (ZnTPP-PDI4)n arrays initiates an ultrafast charge separation that occurs with near unit efficiency to generate a coherent radical ion pair state. One of the spins hops between several closely-coupled electron acceptors.
•Self-assembly of two types of robust perylenediimide chromophores5PDI (red-absorber) and PDI (green absorber) are used to produce an artificial light-harvesting antenna structure that in turn induces self-assembly of a functional special pair that undergoes ultrafast, quantitative charge separation, (5PDI-PDI4)2 .
Wasielewski Group – Winter 2004
Funding: DOE, NSF, ONR, DARPA, PRF
Collaborations: David Tiede and Andrew GosheArgonne National Laboratory